Part VI Flashcards
Treatment in which the source of the therapeutic agent is at a distance from body
Teletherapy
Means distant
Tele
Means therapeia
Treatment
Major drawbacks of Co 60
• High risk of radiation hazard due to radon gas leak produced as a by-product
• High cost of radium
• Large absorption
• Low exposure rate constant
• Low output
invented the cobalt 60 unit in the 1950s
Dr. Harold E. Johns
Main Reasons for its Suitability for Teletherapy
- Availability
- High specific activity
- Sources that minimize the beam penumbra
- Relatively long half-life (5.27 years)
- Almost monochromatic high energy photon
emission (photons of 1.173 MeV and 1.333 MeV in equal quantity).
Fixed head
SSD technique
Picker machine
CO 60 source of about 2000 Ci
Two specifically designed rooms were built to adjacent to each other
1970
Janus CO 60 machine
Isocentric
SSD 60 cm
Output low
Time taken more
Iso-centric CO 60 Therapy machine
Indigenous CO 60 teletherapy machine has a capacity of 200 RMM source and its source to iso-centric distance is 80 cm.
User friendly
Improved radiation safety
Bhabhatron
Main components of teletherapy machine
- Radioactive source
- Source housing including beam collimator and source movement mechanism
- Gantry and stand in isocentric machines
4.patient support assembly - Machine console
Co 60 source structure
Cylindrical double welded stainless steel
1.0 -2.0 cm diameter
Discs/pellets
Circular end facing pt
C0 60 is produced by irradiating ordinary, stable [..] with [..] in the nuclear reactor
59 Co
Neutrons
The decay half life is [..] and the average photon energy is [..]
5.26 years
1.25 MeV
REQUIREMENTS FOR A RADIOACTIVE NUCLIDE TOBE A USEFUL TELETHERAPY SOURCE
- It must emit high gamma rays
- Must have a high specific activity
- Must have a long half-life
- The source must be sealed
- Dual encapsulation
Housing for the source
Source housing
Consists of a steel shell filled with lead for shielding purposes and device for bringing the source in front of an opening in the head from which the useful beam emerges
Source Housing
Off position, [..] is allowed to flow into the space just below the source
On position [..] pumped out to reservoir
Mercury shutter
The source is fixed in front of the aperture and the beam can be turned on and off by a shutter so sitting metal jaws
Moving jaw
Provide beams of desired shape and size rotational movement is continuous and it can your rotate 360°
Collimator
Can rotate 360°, movement is motorized and controlled into directions continuously
Gantry
Treatment bed motorized movements
Horizontal
Vertical
Lateral
Table top (rotation)
Base [rotation]
90 Degrees rotation to each side
110° rotation to each side
x-rays that Constitute the useful treatment beam
Y X-rays
Absorbed in the cobalt metal and stainless steel capsules resulting in the emission of breamsstrahlung and small amount of characteristic x-rays
B particles
Advantages of cobalt 60
- Simplicity of design and ease of operation
- Constancy of beam intensity
- Allows predictability of decay with a well-defined half-life
- It doesn’t have the day-to-day fluctuations typically found in electrical machines
- Cheaper and easy to maintain
Disadvantages of cobalt 60
- Due to its large source, the penumbra is larger than the beam
- The need for source replacement after 2 half-lives (approx.)
- Poor beam flatness for large fields
- emits radiation constantly, thus, the source must be
shielded when the machine is in the beam-off position to protect the personnel
